IMO combining the lines defeats the purpose. The reason that precheck is faster is not usually because you keep your shoes on, it's because people with precheck tend to be frequent travelers and the line moves faster.

Every time I'm stuck in the regular line, it always seems to be behind some people who have apparently been living under a rock for the last couple of decades and think it's ok to have a water bottle in their comically oversized bag, which holds up the line.